I-400 beside submarine tender USS Proteus after the war. Note the large
hangar and forward catapult.(US Navy photo, scanned from Polmar and
Carpenter's "Submarines of the Imperial Japanese Navy")

While Japan built many submarines that were larger than those of other
Navies, the three Sen Toku boats were far larger than anything ever seen
before. Some 60% larger than the largest contemporary American submarine,
USS Argonaut, they had more than twice her range. The most unusual feature
was that they each carried three floatplane bombers (and parts for a fourth), a
feat never achieved by any other class of submarine. These aircraft folded
to fit into the 115-foot cylindrical hangar, which was slightly offset to
starboard and opened forward to access the catapult. The huge double hull
was formed of parallel cylindrical hulls so that it had a peculiar lazy-eight
cross section, and may have inspired the Soviet Typhoon-class built some 40
years later. Although aircraft must be considered their primary armament,
they also carried a formidable torpedo battery and the usual 14cm deck
gun. Anti-aircraft armament included ten 25mm cannons in three triple
mounts and one single. Each of these
boats had radar and a snorkel.

The aircraft were the Aichi M6A1 Seiran,
also carried by the Type AM submarines. Each of these monoplanes could
carry one aerial torpedo or a bomb weighing up to 800kg. Powered by the
1,400hp Atsuta 32 engine (similar to Germany's DB601) they had a top speed of
295mph and were credited with a range of 642 nautical miles. The Sen Toku
submarines carried four aerial torpedoes, three 800kg bombs, and twelve 250kg bombs to
arm these aircraft. These aircraft had their assembly points coated
with fluorescent paint to ease assembly in the dark, so four trained men could
prepare an aircraft for launch in seven minutes. All three aircraft could
be prepared, armed, and launched in 45 minutes.

The
Aichi M6A1 Seiran had the performance of carrier-based bombers, yet it was operated
from a submarine. (Scanned from Rene J. Francillon's
"Japanese Aircraft of the Pacific War")

Unfortunately for
Japan, the war situation deteriorated so rapidly that these boats were never
allowed to show what they could do. On 26 July 1945, I-400 and I-401 set
out on a combat mission to launch their aircraft in Kamikaze attacks on the
American fleet anchorage at Ulithi. In coordination with a Kaiten attack,
they were scheduled to launch early on 17 August, but by then hostilities had
ceased. Both boats therefore returned to Japan and were surrendered to the
Allies. After the war, these two were taken to the United States,
examined, and finally scuttled in the Pacific in 1946. I-402 was converted to carry precious fuel to Japan from the East
Indies, but never performed such a mission. She was scuttled off Goto
Island in 1946.Construction of two further boats of this design, I-404 and I-405,
was
stopped before completion, although I-404 was 90% complete. A further 13
boats were canceled before construction started.